Their direct translation to English would be big-character posters. Wikipedia describes them as handwritten, wall-mounted posters used to protest. These became a method of propaganda in 1966 during the Chinese Cultural Revolution because of how people optimized its powers and made it into propaganda, promoting hateful statements against the Four Olds. Da-zi-baos became so popular and important that even Chairman Mao wrote one of his own on August 5th, 1966, seeking the help of his people to "Bombard the Headquarters" and it was directed towards Liu Shaqoi and Deng Xiaoping, who where the government affairs who calmed the people down in China. After that, da-zi-baos popularity begun to be seen throughout all of China, but for different reasons. One might think that as time passed they would lose their importance, but on the contrary for they were now used as sophisticated debate to satirical entertainment to rabid denunciation; being attacked in a big-character poster was enough to end one's career. This is why when Ji-li sees that her classmates wrote hurtful da-zi-baos about some of her family members, favorite teachers and her she gets so upset and worried.
|Red Guard Source|
The idea of the Red Guards begun in 1966 with a small group of students in Beijing and it's original name was "Chairman Mao's Red Guards." Chairman Mao supported the idea and sanctioned his own groups known as "Red Guards" whose main purpose was to rebel against the system all over China. They would dress in old army jackets and wearing red armbands and would constantly chant "Long Live Chairman Mao!" But bBefore they could commence their servings in the group, it would be mandatory to swear ultimate protection of Mao and his revolutionary line.
Red Guards were unnecessarily violent causing them to turn on each other and that resulting in great numbers of lives lost. The few who survived, and had completed their servings were involuntarily taken from their cities and dragged to the countryside where they would be re-educated by peasants. Many say that the Red Guards were the one of the most violent factors of the Chinese Cultural Revolution.